Conventional techniques for producing relaxed silicon germanium (SiGe) layers on Si substrates typically consist of flowing germane (GeH4) and a silicon precursor, such as silane (SiH4) or dichlorosilane (SiH2Cl2), across a hot semiconductor substrate. When the temperature of these precursors reaches their respective decomposition temperatures, the precursors break down and Si and Ge atoms are free to deposit on the heated substrate. If the substrate is maintained at a sufficiently high temperature, thin-film crystal growth proceeds.
Commercialization of this method to produce relaxed SiGe layers on semiconductor substrates for use in optoelectronic and electronic devices demands an economical process of forming high quality SiGe layers. This means that production costs, such as equipment costs and production time must be minimized, while at the same time, the material properties of the SiGe layers produced must be tailored or optimized for their specific purpose.
Under most conditions, chemical vapor deposition (CVD) processes provide the most economical method of depositing thin layers of crystalline semiconductors. For example, in general, CVD equipment costs are much lower than corresponding equipment costs required to produce the same thin layer product using molecular beam epitaxy (MBE) techniques. Further, high thin-film growth rates can be achieved using CVD. These high growth rates (i.e. greater than 0.1 microns/minute) are essential in producing economical relaxed SiGe materials, since high growth rates reduce deposition time, thereby maximizing production rate and lowering the cost of the product.
While it is important to keep production costs low, it is equally important to produce high quality SiGe layers that possess the appropriate material properties for use in optoelectronic and electronic devices. High quality SiGe films have a low particle defect density (i.e. less than 0.3 particles/cm2) and a low threading dislocation density (i.e. less than 106/cm2). If the produced SiGe layers do not achieve this level of quality, the electronic properties of the layers will not be suitable for use in optoelectronic and electronic devices.
Therefore, commercial production of high quality relaxed SiGe layers depends on the following three criteria:                A high growth rate is needed so that deposition time is minimized and production rate is maximized, thereby reducing costs of producing SiGe layers. An increase in growth rate is typically achieved by increasing the substrate temperature and precursor gas concentration.        The deposited SiGe layer has a low threading dislocation density, so that the deposited SiGe layer is of high quality. A decrease in threading dislocation density is typically achieved by increasing the deposition temperature.        The deposited SiGe layer has a low particle defect density to produce a high quality SiGe layer. A major source of particle defects during epitaxial deposition is flaking of deposits on a reactor wall. Therefore a decrease in reactor wall coating buildup generally decreases the number of particles that subsequently deposit on the SiGe layers.        
One of the problems encountered when trying to extend conventional research methods of producing SiGe layers to a commercially viable process is that efforts to increase the growth rate and decrease the threading dislocation density also increase the particle defect density. This occurs because an increase in deposition temperature and an increase in precursor concentration leads to particle defects nucleating and depositing on the substrates and to an increase in coating buildup on the reactor walls, which eventually leads to flaking of the coating during growth of the SiGe layers.
This problem is exacerbated by the use of germane gas as a precursor. Germane decomposes at a much lower temperature, 675° C., than its silicon precursor counterpart (i.e. 900° C. for silane gas and 950° C. for dichlorosilane). Thus, any increase in deposition temperature increases the amount of Ge atoms available for deposition and thus increases deposits on reactor walls and particle defects on the substrate. The result of high temperature growth with germane is the formation of a brown or black, partially opaque coating on an inner surface of the reactor. In the case of a typical lamp-heated production CVD reactor, this coating limits the transmission of radiated energy from the lamps through the quartz reactor wall to the substrate. The loss of transmissivity of the reactor, which is typically a quartz tube, causes the inner surface of the reactor to heat to a temperature above 700° C., resulting in further deposits. Eventually, if left unchecked, the temperature of the reactor wall will reach 1000° C., at which point devitrification occurs. Both the reactor coating and devitrification can flake from the reactor walls and deposit on the substrate.
Most applications for relaxed Si1-xGex layers require an atomic Ge content above about 15% (x=0.15). In order to increase the Ge content in the layer, the precursor concentration of germane must be increased, thereby increasing particle defect generation.
Thus, it would be desirable to develop a method of producing relaxed SiGe layers on a semiconductor substrate that minimizes particle defect generation during deposition of SiGe layers, especially for high-Ge-content SiGe layers.